Fiber optic patch-panels are used to terminate large numbers of optical fibers at an array of connectors mounted on modular plates, thereby providing a location to manually interconnect patch cords for their routing to adjacent circuits. Splice trays within the panel retain slack fiber and the splices joining connector pigtails to the individual fiber elements originating from one or more cables. Typical patch-panel systems interconnect 100 to 10,000 fibers. Connection to various types of transmission equipment, such as transceivers, amplifiers, switches and to outside plant cables destined for other exchanges, local offices, central offices, optical line terminations and points-of-presence are configured manually at the patch-panel.
As the reach of fiber optic systems extends to FTTH (Fiber-to-the-Home), access and enterprise networks, the locations of patch-panels are becoming geographically more dispersed and the sheer numbers of ports are increasing dramatically. Consequently, the tasks of allocating, reconfiguring and testing a fiber circuit within the network is challenging because of the potential for errors or damage resulting from manual intervention. Remotely reconfigurable patch-panels reduce the operational and maintenance costs of the network, improve the delivery of new services to customers and leverage costly test and diagnostic equipment by switching or sharing it across the entire network. Therefore, it is appealing from a cost, accuracy and response-time perspective to configure the patch-panel from a remote network management center. The key building block of an automated patch-panel system is a scalable, high port count, all-optical cross-connect switch.
A wide range of technologies has been developed to provide optical cross-connect functionality with several hundred ports. These include arrays of steerable micro-electromechanical (MEMS) mirrors to deflect beams, piezoelectric steerable collimators that direct free space beams between any pair of fibers, and complex robotic cross-connects utilizing actuators that reconfigure fiber optic connections. For the purpose of comparison we shall categorize the first two approaches as “non-robotic” and the latter approach as “robotic”.
Non-robotic cross-connect switches, while offering the potential for relatively high speed (10 ms), do so at the expense of limited optical performance and scalability. The coupling of light into and out-of fiber and free-space introduces substantial alignment complexity and significantly increases insertion loss, back reflection and crosstalk. These approaches also require power to maintain active alignment and introduce micro-modulation of the transmitted signal as a result of the need to actively maintain mirror alignment. As a consequence, MEMS switches do not provide an optically transparent, plug-and-play replacement for manual fiber optic patch-panels.
Robotic cross-connect approaches perform substantially better from the standpoint of optical performance and their ability to maintain signal transmission even in the absence of power. However, the scalability of such approaches has been limited. The footprint of prior art robotic switch designs scales as N2, where N is the number of circuits. The size of the switch matrix is typically N columns by N rows wide with N2 possible interconnection points. Considering that the central offices of today's telecommunications service providers already utilize 1000 to 10,000 port patch panels, scalability is of prime importance. Therefore, an approach scaling linearly in N would enable the cross-connect to achieve a substantially higher port density commensurate with manual patch-panels.
Moreover, typical network installations are performed in an incremental fashion, whereby fiber circuits are added to the system as needed. Robotic and non-robotic approaches have not been modular and as such, they do not offer an upgrade path from 200 ports to 1000 ports, for example. To achieve port counts above several hundred, a three-stage Clos network interconnection scheme must be implemented [C. Clos, “A study of non-blocking switching networks” Bell System Technical Journal 32 (5) pp. 406-424 (1953)], leading to a substantial increase in cost, complexity and a reduction in optical performance by virtue of the need to transmit through a series arrangement of three rather than one switch element.
In addition, the optical performance of robotic cross-connects, while improving on non-robotic approaches, is still inferior to manual patch-panels because they introduce an additional fiber optic connection in series with each fiber circuit. A manual patch-panel requires only one connector per circuit and offers a typical loss of <0.25 dB, while the equivalent robotic patch-panel incorporates at least two connectors per circuit. This increases the loss by a factor of 2.
Furthermore, robotic approaches have required significant numbers of precision, miniature translation stages (2N) and at least 4 precision robotic actuators to align large numbers of input and output fiber end faces to one another. These fiber end faces physically contact one another and can exhibit wear-out for switch cycles in excess of 1000, or can become damaged at the high optical power levels transmitted through fiber in Raman amplified systems. The performance of frequently reconfigured test ports is therefore susceptible to degradation.
The prior art describes various mechanical approaches to interconnecting a number of fibers. U.S. Pat. No. 5,699,463 by Yang et al. discloses a mechanical optical switch for coupling 1 input into N outputs by translating an input fiber and lens to align to a particular output fiber. For patch-panel applications, the required number of input and output ports are near-symmetrical and equal to N.
A series of patents by Lucent, NTT and Sumitomo disclose various implementations of large port count optical cross-connects in which fiber optic connections are reconfigured by a robotic fiber handler. For example, Goossen describes a switch utilizing a circular fiber bundle and a circular ferrule loader ring in U.S. Pat. No. 6,307,983. Also, U.S. Pat. No. 5,613,021, entitled “Optical Fiber Switching Device Having One Of A Robot Mechanism And An Optical Fiber Length Adjustment Unit” by Saito et al., describes the use of a robotic fiber handler to mechanically reconfigure connectors on a coupling board. U.S. Pat. No. 5,784,515, entitled “Optical Fiber Cross Connection Apparatus and Method” by Tamaru et al. describes a switch in which connectorized optical fibers are exchanged between an “arrangement board” and a “connection board” by a mechanized fiber handler. A motorized means of fiber payout is further described. Related approaches are described in a series of patents including JP7333530, JP2003139967, JP2005346003, JP11142674, JP11142674, JP10051815 and JP7104201.
To overcome the prior art's susceptibility to fiber entanglement, Sjolinder describes an approach to independently translate fiber connectors along separate, linear paths in two spaced-apart planes on opposite sides of an honeycomb interface plate [“Mechanical Optical Fibre Cross Connect” (Proc. Photon. Switching, PFA4, Salt Lake City, Utah, March 1995]. In the first active switch plane, N linearly translating connectors are driven along spaced-apart rows by actuators and in the second active switch plane, an additional N linearly translating connectors are driven along spaced-apart columns. Row and column actuators are configured perpendicular to one another. Connections are made between fiber pairs located in any row and in any column by mating connectors at any of the N2 common insertion points within the interface plate. This approach requires at least 2N actuators to arbitrarily connect N inputs with N outputs.
An extension of this cross-connect approach is disclosed in U.S. Pat. No. 6,859,575 by Arol et al., U.S. Pat. No. 6,961,486 by Lemoff et al. and WO2006054279A1 by J. Arol et al. They describe robotic cross-connect switches comprised of N input optical fibers supported by N translation stages and M output fibers supported by M translation stages. Each input fiber requires a shared or dedicated mechanical actuator to linearly translate both parallel to (x,y) and perpendicular to (z) the switch active planes. The connectors require individual z translation to physically contact the opposing facets of aligned input and output fibers.
The robotic cross-connect approaches described in the prior art have limited scalability and optical performance. The application of robotic optical switches to fiber optic patch panels demands true optical transparency, scalability to port counts in excess of 1000 within the footprint of a manual patch panel and modularity and the ability to incrementally add circuits on an as-needed basis.